The present invention relates to a micro metal-wiring structure used in a large-scale integrated circuit.
Aluminum wiring is widely used for a large-scale integrated circuit as characteristics of aluminum are suitable for micro wiring construction. An example of a conventional aluminum wiring construction is shown in FIGS. 1A and 1B. Since the width of the aluminum wiring has become very small, aluminum grains 101 are arranged along the extending direction of the wiring such that the grain boundary lies perpendicular to the extending direction of the wiring and crosses the entire width of the wiring. This structure is referred to as a bamboo structure. As reported in Japanese metal society report vol.28, No.1, p40-47, in this bamboo structure, resistance against an electro-migration (to be defined later) is improved but resistance against a stress-induced-migration (to be defined later) is decreased.
Electro-migration is a phenomenon that occurs in a metal wiring when the wiring is kept under a high temperature and a high current density. Under such conditions, a void may be locally generated and the void then growing to become a large defect; results in an increase in the electric resistance or in an occurrence of an open failure. The generation of a void is caused by a migration of the metal atoms due to movement of electrons. The void is usually generated along the grain boundary.
Stress-induced-migration is a phenomenon that occurs in the grain boundary of aluminum under only a high temperature condition. A void is generated due to a stress caused by thermal expansion differences between a silicon substrate (Si), an aluminum alloy, and an insulating layer surrounding the aluminum alloy. The linear expansion coefficient of silicon is 2.5.times.10.sup.-6 K.sup.-1, of aluminum alloy 25.times.10.sup.-6 K.sup.-1, and of the insulation layer only a few 10.sup.-6 K.sup.-1. The thermal expansion of the aluminum alloy differs from that of the other materials by one or two orders of magnitude. Due to these thermal expansion differences, a stress is exerted on the grain boundary of the aluminum wiring, and hence it is said that a void occurs in the grain boundary due to stress. The occurrence of a void results in fracture of the wiring. In the bamboo structure, since the cross section of the grain boundary is small and the boundary crosses the extending direction of the wiring, wire fracture occurs easily due to stress-induced-migration. A solution to this problem is desirable.
In order to decrease the possibility of a wire fracture due to stress-induced-migration, adding copper (Cu) or paradium (Pd) to Al--Si alloy was suggested in J.Vac.Sci.Technol.B8(6),Nov/Dec 1990, P.1232. This technical paper describes that segregation of Cu or Pd in the Al grain boundary, increases a bonding force of the grains. Additionally, there has been suggested that provision of transition elements, or silicide or nitride of transition elements, which have a high melting point, between layers decreases a wire fracture rate.
However, since, further integration of the large-scale integrated circuit is desirable, and micronization of wiring has been further advanced, a decrease in the service life of wiring due to stress-induced-migration has become an important problem.
In order to obtain a reliable wiring, it is required to increase an amount of supplemental metal, however, the increase in amount of supplemental metal increases a resistance of the wiring. Additionally, segregation of the supplemental metal increases possibilities of boundary corrosion and increases occurrence of formation of undesirable remaining portions formed during the etching process. Further, there is a problem associated with target manufacturing when using a different metal to be added.
Furthermore, with regard to stacking of the material, under conditions where constant thickness of the wiring is maintained, layers other than the Al layer should be made thick so as to maintain reliability. However, since materials having a high melting point, such as tungsten (W), have also a high resistance compared to that of aluminum (Al), an increase of the effective resistance of the wiring is not avoided. Conversely, in a condition where a constant electric resistance is maintained, the wiring should be made thick so as to maintain an appropriate current path, and thus the difference in heights between the wire and the substrate becomes large resulting in difficulty of planarization when applying an insulating material over the wiring and over the surface of the substrate.
Accordingly, in micro metal-wiring construction used in a large-scale integrated circuit, it is required to implement measures for stress-induced-migration so as to avoid an increase of the electric resistance of wiring while maintaining electro-migration at a low level.